The field of the invention is magnetic resonance imaging (“MRI”) of extended fields of view, and particularly MR angiography (“MRA”), studies of the human vasculature using contrast agents which enhance the NMR signals.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins, and after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals, or “views” are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR angiography (MRA) is the application of magnetic resonance imaging methods to the depiction of the human vasculature. To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. Excellent diagnostic images may be acquired using contrast-enhanced MRA if the data acquisition is properly timed with the bolus passage.
The non-invasiveness of MRA makes it a valuable screening tool for cardiovascular diseases. Screening typically requires imaging vessels in a large volume. This is particularly true for diseases in the runoff vessels of the lower extremity. The field of view (FOV) in MR imaging is limited by the volume of the B0 field homogeneity and the receiver coil size (typically, the FOV<48 cm on current commercial MR scanners). The anatomic region of interest in the lower extremity, for example, is about 100 cm and this requires several FOVs, or stations, for a complete study. This requires that the patient be repositioned inside the bore of the magnet, the patient be re-landmarked, scout images be acquired and a preparation scan be performed for each FOV. All of these additional steps take time and, therefore, are expensive. When contrast enhanced MRA is performed, the repositioning also necessitates additional contrast injections.
Recently gadolinium-enhanced bolus chase techniques have been reported which overcome this difficulty, K. Y. Ho, T. Leiner, M. H. de Hann, J. M. A. van Engleshoven, “Gadolinium optimized tracking technique: a new MRA technique for imaging the peripheral vascular tree from aorta to the foot using one bolus of gadolinium (abs).” Proc. 5th Meeting of ISMRM, p 203, 1997. As described in U.S. Pat. Nos. 5,924,987 and 5,928,148, MRA data is acquired from a large field of view by automatically moving the patient table to a plurality of different locations during the scan and acquiring an image at each station. The movement of the table may be timed to follow the contrast bolus through the vasculature so that peak contrast is achieved at each station.
The result of prior bolus chase MRA methods is that one ends up with a plurality of images. These are manually or automatically registered with each other to provide a single image that covers the entire extended field of view. One difficulty with this approach, however, is that the separate images have different brightnesses and/or contrasts. As a result, there are discontinuities at the boundaries of images where they have been patched together. Another difficulty with the multi-station method is that valuable time is lost when the table is moved from one station to the next. During that time no image data is being acquired and further time is lost in bringing the spin magnetization into dynamic equilibrium before image data is acquired. In a three-dimensional scan this lost data acquisition time can mean that the scanning process does not keep pace with the moving contrast bolus and some image contrast is lost in later images.
As described in co-pending U.S. patent application Ser. No. 09/993,120 filed on Nov. 26, 2001 and entitled “Method For Acquiring MRI Data From A Large Field Of view Using Continuous Table Motion”, one can also acquire MRI data from an extended field of view in one continuous scan. With this method the patient table is in continuous motion and the phase encodings along y (and along z if 3D) are repeatedly cycled during the scan. One large image over the extended longitudinal FOV is reconstructed from the acquired MRI data.
Artifacts in the form of periodic signal falloff have been observed in MR images acquired according to this continuous table motion method. Referring to FIG. 7, which is an MR image of an agar gel phantom, the artifacts can be seen as dark regions where the signal has fallen off. In this example, the extended longitudinal FOV of the scan is 65 cm and the artifacts appear at 25 cm intervals in the image of the phantom. The 25 cm intervals between artifacts corresponds to the scanner's longitudinal FOV.